Cathode and battery including same

ABSTRACT

A cathode and a battery including the cathode are provided. The cathode includes a cathode mixture layer with a cathode active material and a binder. The binder can include, for example, a synthetic rubber latex and a thickener, polyvinylidene fluoride denaturalized by maleic acid and/or the like.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority to Japanese Patent DocumentNo. P2002-215168 filed on Jul. 24, 2002, the disclosure of which isherein incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to cathodes and batteries includingsame. More specifically, the present invention relates to cathodeshaving a cathode mixture layer including a cathode active material and abinder, and batteries including the cathode.

[0003] Recently, in connection with the development of electronicengineering, many compact portable electronic devices, such as acombination camera (e.g., video tape recorder), a mobile phone, and alaptop computer are commonly known and used, and the size and weight ofsuch devices are being reduced. Consequently, as a portable power sourceused to power save, a compact and lightweight battery with a high energydensity, particularly, a secondary battery has been developed.

[0004] For example, a secondary battery having an anode active materialcapable of inserting and extracting lithium metals, lithium compounds,or lithium ions, has the high voltage and excellent reversibility. Inparticular, a lithium ion secondary battery, using a composite oxide oflithium and a transition metal as a cathode active material, and using acarbonaceous material as an anode active material, is lightweight andhas a large discharge capacity, compared to conventional lead secondarybatteries and nickel-cadmium secondary batteries. Thus, the lithium ionsecondary battery is widely used for electronic devices, such as mobilephones, laptops and the like.

[0005] Currently, a primary example of typically used cathode activematerials for the lithium ion secondary battery is LiCoO₂. There exist,however, a number of problems related to use of same, such as, in termsof load characteristics, charge and discharge cycle characteristics, andsafety or the like. For example, in order to improve the loadcharacteristics, it is necessary to smooth an electrode, further to makethe electrode into a thin film. To obtain such electrode, it isnecessary to downsize grain diameters of the materials making theelectrode, and to improve conductivity. However, when downsizing thegrain diameters, the specific surface area becomes large. Thus, unlessmore binder is added, the electrode becomes fragile, and sufficient peelstrength cannot be obtained.

[0006] However, polyvinylidene fluoride (PVDF), which has beenconventionally and primarily used as a binder, is a non-electricallyconductive polymer. Therefore, there exists a problem such thatincreasing the amount of PVDF causes not only lowering of a ratio of anactive material in the electrode and lowering of a charge and dischargecapacity, but also hindrance of electron transfer, increase of internalresistance of the electrode, and significant deterioration of charge anddischarge cycle life of the battery and capability of high load chargeand discharge of the battery. Further, there exists another problem suchthat the electrode becomes hard and fragile, and electrode peeling andcracking occur.

[0007] A need therefore exists to provide improved batteries, includingparts thereof, such as cathodes.

SUMMARY OF THE INVENTION

[0008] The present invention provides a cathode, wherein a high-strengththin film electrode can be realized, its load characteristics areimproved, its charge and discharge capacity and capacity maintenanceratio are high, and its charge and discharge cycle life are enhanced,and a battery including same.

[0009] A cathode according to an embodiment of the present inventionincludes a cathode mixture layer containing a cathode active materialand a binder. The cathode mixture layer contains a synthetic rubberlatex adhesive and a thickener as the binder. In the cathode mixturelayer, the content of the synthetic rubber latex adhesive ranges fromabout 2 wt % to about 4 wt %, and the content of the thickener rangesfrom about 0.5 wt % to about 2.5 wt %.

[0010] A cathode according to an embodiment of the present inventioncomprises a cathode mixture layer containing a cathode active materialand a binder. The cathode mixture layer contains polyvinylidene fluoridedenaturalized by maleic acid as the binder, wherein the content of thepolyvinylidene flouride in the cathode mixture layer ranges from about0.5 wt % to about 4 wt %.

[0011] A battery according to an embodiment the present inventionincludes a cathode, an anode, and an electrolyte. The cathode includes acathode mixture layer containing a cathode active material, andsynthetic rubber latex adhesive and a thickener as the binder. In thecathode mixture layer, the content of the synthetic rubber latexadhesive ranges from about 2 wt % to about 4 wt %, the content of thethickener ranges from about 0.5 wt % to about 2.5 wt %, and a chargefinal voltage is about 4.0 V and under.

[0012] A battery according to an embodiment of the present inventionincludes a cathode, an anode and an electrolyte. The cathode includes acathode active material and polyvinylidene fluoride denatured by maleicacid as the binder. In the cathode mixture layer, the content ofpolyvinylidene fluoride denatured by maleic acid ranges from about 0.5wt % to about 4 wt %, and a charge final voltage is about 4.0 V andunder.

[0013] In the cathode, according to an embodiment of the presentinvention, since the synthetic rubber latex adhesive and the thickenerare included as a binder, high flexibility and smoothness can beobtained. Thus, electrode peeling and cracking are prevented. Further,the synthetic rubber latex adhesive and the thickener, or polyvinylidenefluoride denaturalized by maleic acid is contained as a binder accordingto an embodiment of the present invention. Thus, the content of thebinder can be decreased, and the ratio and capacity of the activematerial can be increased. Further, electron transfer can be facilitatedto decrease the resistance.

[0014] In a battery according to an embodiment of the present invention,a cathode according to an embodiment of the present invention isemployed. Thus, the charge and discharge capacity is increased and theinternal resistance is decreased. Further, the charge and dischargecapacity and the charge and discharge cycle life are improved, and theload characteristics are improved.

[0015] Additional features and advantages of the present invention aredescribed in, and will apparent from, the following Detailed Descriptionof the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

[0016]FIG. 1 is an exploded perspective view of a secondary batteryaccording to an embodiment of the invention.

[0017]FIG. 2 is a cross sectional view taken along line I-I of a batteryelement illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention generally relates to batteries and partsthereof. More specifically, the present invention relates to cathodesthat have a cathode mixture layer that includes a cathode activematerial and a binder, and batteries including same.

[0019]FIG. 1 shows an exploded view of a secondary battery according toan embodiment of the invention. FIG. 2 shows a cross sectional viewtaken along line I-I of a battery element 20 illustrated in FIG. 1. Thepresent invention will be described below, without limitation, ingreater detail with reference made to the figures where appropriate.

[0020] In a secondary battery according to an embodiment of the presentinvention, the battery element 20, wherein a cathode 21 and an anode 22are layered and wound with an electrolyte layer 23 and a separator 24 inbetween, is crimped and enclosed in film exterior members 30 a and 30 b.The exterior members 30 a and 30 b are made of, for example, an aluminumlaminated film wherein a polyolefin film, an aluminum foil, and apolyolefin film are applied together in this order. Outer edges of theexterior members 30 a and 30 b are contacted with each other. An end ofa cathode lead wire 11 and an end of an anode lead wire 12 are projectedfrom the exterior members 30 a and 30 b. Adhesive film 31 is insertedbetween the exterior members 30 a and 30 b and the cathode lead wire 11and the anode lead wire 12, for example. The adhesive film 31 is used tosecure the insulation, as well as to improve the adhesion propertiesbetween the cathode lead wire 11 and the anode lead wire 12 and theexterior members 30 a and 30 b.

[0021] In the battery element 20 according to an embodiment of thepresent invention, for example, the separator 24, the electrolyte layer23, the cathode 21, the electrolyte layer 23, the separator 24, theelectrolyte layer 23, the anode 22, and the electrolyte layer 23 aresequentially layered and wound. At the outermost circumferential part,for example, a protective tape 25 is adhered. The cathode lead wire 11is connected to the cathode 21 of the battery element 20, and the anodelead wire 12 is connected to the anode 22. The cathode lead wire 11 andthe anode lead wire 12 can be respectively made of a metal or an alloyhaving conduction. For example, it is preferable that the cathode leadwire 11 is made of aluminum and the anode lead wire 12 is made ofnickel. It should be appreciated that other suitable materials orcombinations thereof can be used.

[0022] The cathode 21 is, for example, composed of a cathode currentcollector layer 21 a and cathode mixture layer 21 b, having a structurewherein cathode mixture layer(s) 21 b is provided on both sides orsingle side of the cathode current collector layer 21 a. The cathodecurrent collector layer 21 a is made of metal foil, such as aluminumfoil, nickel foil, stainless foil, the like or combinations thereof. Thecathode mixture layer 21 b contains, for example, a cathode activematerial such as lithium phosphorous oxide or the like and a binder adescribed below, for example, and may additionally contain a conductiveagent. Further, the cathode mixture layer 21 b is not provided on oneend part of the cathode current collector layer 21 a, so that the endpart is exposed. The cathode lead wire 11 is attached to the exposed endpart.

[0023] The lithium phosphorous oxide has, for example, an olivinestructure. It contains, in an embodiment, at least one first element,such as manganese (Mn), chromium (Cr), cobalt (Co), copper (Cu), nickel(Ni), vanadium (V), molybdenum (Mo), titanium (Ti), zinc (Zn), aluminum(Al), gallium (Ga), magnesium (Mg), boron (B), niobium (Nb), iron (Fe),and the like; lithium; phosphorous; oxygen and the like and combinationsthereof. The lithium phosphorous oxide is expressed by, for example, achemical formula of Li_(x)MPO₄ (0<x≦1.2), wherein M represents the firstelement. Examples include LiFe_(0.2)Cu_(0.8)PO₄, LiFe_(0.9)Ti_(0.1)PO₄,LiFe_(0.8)Zn_(0.2)PO₄, LiFe_(0.8)Mg_(0.2)PO₄, the like and combinationsthereof. In particular, lithium-iron-phosphorous composite oxides, whichis readily available and inexpensive, are preferable.

[0024] The lithium phosphorous oxide provides excellent characteristicsin the case where a charge final voltage is controlled to be about 4.0 Vor less. Binders described later start to be decomposed when the chargefinal voltage exceeds about 4.0 V. However, in the case where thelithium phosphorous oxide is used as a cathode active material and thecharge final voltage is controlled to be about 4.0 V or less, highbattery characteristics can be obtained, which is preferable. As acathode active material, other materials capable of controlling thecharge final voltage to be about 4.0 V or less can be used. It is alsopossible to combine such materials with the lithium phosphorous oxide.

[0025] An average grain diameter of the cathode active material is, forexample, preferably in the range from about 0.5 μm to about 3 μm. Inthis regard, it is believed that the cathode 21 can be smoothed and madeinto a thin film by downsizing the average grain diameter. Further, byusing the binders described later, even when the average grain diameteris downsized, the flexible and smooth cathode 21 can be obtained withoutincreasing a ratio of the binder.

[0026] It is preferable to use, for example, a synthetic rubber latexadhesive and a thickener as a binder. It is believed that by using them,the flexible and smooth cathode 21 can be obtained, and electrodepeeling and cracking can be prevented. Further, it becomes possible toreduce the amount of the binder compared to in conventional cases, andto increase the ratio of amount of the cathode active material.Moreover, electron transfer can be facilitated, and the internalresistance of the battery can be reduced.

[0027] Examples of the synthetic rubber latex adhesive are styrenebutadiene rubber latex, nitrile-butadiene rubber latex, methylmethacrylate butadiene rubber latex, chloroprene rubber latex and thelike, or the like. Any one of them or a mixture of two or more of themmay be used. Examples of the thickener are synthetic polymers, such aspolyacrylic acid, polyethylene oxide, polyvinyl alcohol, polyacrylamide;cellulose ether resins such as methyl cellulose, ethyl cellulose,triethyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose,aminoethyl cellulose; or natrium salt type or ammonium salt type ofthese cellulose ether resins. Any one of them or mixture of two or moreof them may be used. It is preferable to use ammonium salt typepolyacrylic acid with salt tolerance among the above materials,considering it is relatively inexpensive and easy to use.

[0028] It is preferable that, the content of the synthetic rubber latexadhesive in the cathode mixture layer 21 b, ranges from about 2 wt % toabout 4 wt % with respect to all mass of the cathode mixture layer 21 b;and the content of the thickener in the cathode mixture layer 21 branges from about 0.5 wt % to about 2.5 wt % with respect to all mass ofthe cathode mixture layer 21 b. When the content of the synthetic rubberlatex adhesive exceeds the above range, viscosity is remarkably raisedin forming the cathode mixture layer 21 b, so that application to thecathode current collector layer 21 a becomes difficult; and when thecontent of the synthetic rubber latex adhesive is below theabove-described range, the cathode mixture layer 21 b becomes fragile,so that sufficient strength cannot be obtained. When the content of thethickener exceeds the above range, gelation is significant in formingthe cathode mixture layer 21 b, so that application to the cathodecurrent collector layer 21 a becomes impossible; and when the content ofthe thickener becomes less, the cathode mixture layer 21 b becomefragile, so that sufficient strength cannot be obtained.

[0029] It is also preferable to use polyvinylidene fluoridedenaturalized by maleic acid (hereinafter referred to as maleicacid-denaturalized polyvinylidene fluoride) as another binder accordingto an embodiment of the present invention. It is believed that, by usingthis, an amount of adhesive can also be reduced compared to inconventional cases, since peel strength is improved. A maleicacid-denaturalized amount preferably ranges from about 0.1 wt % to about0.4 wt %. When a denaturalized amount exceeds the above range, gelationis significant in forming the cathode mixture layer 21 b, so thatapplication to the cathode current collector layer 21 a becomesimpossible; and when a denaturalized amount becomes less, the cathodemixture layer 21 b become fragile, so that sufficient strength cannot beobtained.

[0030] Further, it is also preferable to use a material obtained bysubstituting part of maleic acid-denaturalized polyvinylidene fluoridewith hexafluoro propylene (HFPr) (hereinafter referred to asHFPr-substituted maleic acid-denaturalized polyvinylidene fluoride) as abinder. It is believed that the application aspect of the cathodemixture is improved, a higher discharge capacity can be obtained, andthe cycle characteristics can be improved. Substitution ratio ofhexafluoro propylene is preferably about 5 wt % and under, since whenthe ratio exceeds 5 wt %, though peel strength can be obtained,absorption of electrolyte solution is intense, peeling due to charge anddischarge is easy to occur, and cycle life is easy to lower.

[0031] It is preferable that a content of maleic acid-denaturalizedpolyvinylidene fluoride or HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride in the cathode mixture, in an embodiment, rangesfrom about 0.5 wt % to about 4 wt %. When a content exceeds the aboverange, gelation in forming the cathode mixture layer 21 b issignificant, so that application to the cathode current collector layer21 a becomes impossible; and when a content becomes less, the cathodemixture layer 21 b become fragile, and sufficient strength cannot beobtained.

[0032] Examples of conductive agents are carbon materials such as carbonblack, e.g. Ketjen black, graphite the like or combinations thereof.Such carbon material is preferably contained within the cathode activematerial, and the content in an embodiment preferably ranges from about5 wt % to about 12 wt % with respect to the total amount of the cathodeactive material and the carbon material. When the content is less thanabout 5 wt %, the conductivity decreases, and significant deteriorationof the load characteristics and deterioration of charge and dischargecapacity occur; and when the content exceeds about 12 wt %, the ratiobecomes excess in relation to the cathode active material, bulk densityof the cathode mixture layer 21 b is large, and further increasingcontent of the binder becomes necessary, both of which are notpreferable.

[0033] The anode 22 has a structure, in an embodiment, wherein an anodemixture layer(s) 22 b is provided on both sides or single side of ananode current collector layer 22 a respectively in a manner similar toin the cathode 21. The anode current collector layer 22 a is made ofmetal foil, such as copper foil, nickel foil, stainless foil and/or thelike. The anode mixture layer 22 b contains, for example, any one kindor two or more kinds of anode materials capable of inserting andextracting lithium as the electrode active material, and mayadditionally contain a binder such as polyvinylidene fluoride asnecessary. The anode mixture layer 22 b are not provided on one end partof the anode current collector layer 22 a, so that the end part isexposed. The anode lead wire 12 is attached to this exposed end part.

[0034] Examples of anode materials capable of inserting and extractinglithium are carbon materials, metal oxides, high molecular weightmaterials and the like. As the carbon materials, for example, there arepyrolytic carbons, cokes, graphite, glassy carbons, organic highmolecular weight compound fired bodies, carbon fibers, sphericalcarbons, or activated carbons and the like. The cokes include pitchcokes, needle cokes, petroleum cokes and the like. The organic highmolecular weight compound fired bodies denote ones obtained by firingand carbonizing high molecular weight materials such as a phenol resin,a furan resin or the like at appropriate temperature. The carbon fibersinclude a mesophase carbon fiber or the like. The spherical carbonsinclude mesophase carbon micro beads or the like. Examples of the metaloxides are iron oxide, ruthenium oxide, or molybdenum oxide or the like.Examples of the high molecular weight materials are polyacetylene,polypyrrole, or the like.

[0035] As an anode material capable of inserting and extracting lithium,substances, alloys, or compounds of metal elements or semimetalelements, which are capable of forming alloys with lithium. The alloysinclude, in an embodiment, two or more metal elements, and, in addition,alloy-based materials that include one or more metal elements and one ormore semimetal elements. In the structure of each of the materials,solid solution, eutectic (eutectic mixture), or intermetallic compoundexists, or two or more of them coexist.

[0036] Examples of such metal elements or semimetal elements include tin(Sn), lead (Pb), aluminum (Al), indium (In), silicon (Si), zinc (Zn),antimony (Sb), bismuth (Bi), gallium (Ga), germanium (Ge), arsenic (As),silver (Ag), hafnium (Hf), zirconium (Zr), and yttrium (Y) and the like.An alloy or compound of these elements is expressed by, for example, achemical formula of M_(s)Mb_(t)Li_(u), or a chemical formula ofMa_(p)Mc_(q)Md_(r). In an embodiment, Ma represents at least one ofmetal elements and semimetal elements capable of forming an alloy withlithium, Mb represents at least one of metal elements and semimetalelements other then lithium and Ma, Mc represents at least one ofnon-metal elements, and Md represents at least one of metal elements andsemimetal elements other than Ma. The values of s, t, u, p, q, and rsatisfy s>0, t≧0, p>0, q>0, and r≧0, respectively.

[0037] In an embodiment, a substance, an alloy, or a compound of a group4B metal element(s) and/or semimetal element(s) is preferable. In anembodiment, silicon and tin, and their alloys and compounds arepreferred in crystalline or amorphous state.

[0038] Examples of such alloys and compounds are LiAl, AlSb, CuMgSb,SiB₄, SiB₆, Mg₂Si, Mg₂Sn, Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂,CrSi₂, CU₅Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC,Si₃N₂O, Si_(v) (0<v≦2), SnO_(w) (0<v≦2), SnSiO₃, LiSiO, LiSnO and thelike.

[0039] Any one kind or mixture of two or more of the above can be usedfor an anode material capable of inserting and extracting lithiumaccording to an embodiment of the present invention.

[0040] The electrolyte layer 23 includes, in an embodiment, anelectrolyte solution wherein the lithium salt as the electrolyte salt isdissolved in a nonaqueous solvent; and a gel electrolyte containing ahigh molecular weight material.

[0041] Examples of the lithium salts include LiPF₆, LiBF₄, LiClO₄,LiCF₃SO₃, LiN (CF₃SO₂)₂, LiN (C₂F₅SO₂)₂ and the like. One of them, ormixture of two or more of them can be used.

[0042] The nonaqueous solvents include, for example, ethylene carbonate,propylene carbonate, butylene carbonate, γ-butyrolactone,γ-valerolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane,tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxyline, methylacetate,methyl propionate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, 2,4-difluoro anisole, 2,6-difluoro anisole, 4-bromoveratroleand the like. One or mixture of two or more of the nonaqueous solventscan be used.

[0043] The high molecular weight materials include polyvinylidenefluoride, polyacrylonitrile, polyethylene oxide, orpolymethacrylonitrile and the like. One of them, or mixture of two ormore of them can be used corresponding to type of usage.

[0044] As the separator 24, for example, an insulative thin film havinglarge ion permeability and a given mechanical strength is used.Specifically, a porous thin film made of a polyolefin material, such aspolypropylene, polyethylene and the like, or a porous thin film made ofan inorganic material such as a ceramic non woven cloth is used. It ispossible to layer two or more kinds of such porous thin films. Athickness of the separator 24 preferably ranges from about 1 μm to about30 μm, considering the mechanical strength and the battery capacity.

[0045] In an embodiment, a secondary battery can be produced asdescribed below.

[0046] First, for example, a cathode active material, a binder, and aconductive agent as necessary are mixed. The mixture is dispersed in asolvent such as N-methyl-2-pyrrolidone to thereby obtain the cathodemixture slurry. As a binder, as mentioned above, the synthetic rubberlatex adhesive and the thickener, or maleic acid-denaturalizedpolyvinylidene fluoride, or HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride can be used in an embodiment. After producingthe cathode mixture slurry, for example, the cathode mixture slurry isapplied on both sides or single side of the cathode current collectorlayer 21 a, dried, and compression molded to form the cathode mixturelayer 21 b. In such a manner, the cathode 21 is produced. Then, thecathode mixture slurry is not applied on one end part of the cathodecurrent collector layer 21 a, and the end part is exposed.

[0047] Subsequently, for example, an anode active material, a binder,and a conductive agent as necessary are mixed to prepare an anodemixture. The mixture is dispersed in a solvent such asN-methyl-2-pyrolidone to thereby obtain the anode mixture slurry. Afterproducing the anode mixture slurry, for example, this anode mixtureslurry is applied on both sides or single side of the anode currentcollector layer 22 a, dried, and compression molded to form the anodemixture layer 22 b. In such a manner, the anode 22 is produced. Then,the anode mixture slurry is not applied on one end part of the anodecurrent collector layer 22 a, and the end part is exposed.

[0048] After producing the cathode 21 and the anode 22, for example, thecathode lead wire 11 is attached to the exposed part of the cathodecurrent collector layer 21 a, and the anode lead wire 12 is attached tothe exposed part of the anode current collector layer 22 a respectivelyby resistance welding, ultrasonic welding or the like.

[0049] Subsequently, for example, the electrolyte layer 23 made of a gelelectrolyte is formed on the cathode mixture layer 21 b and the anodemixture layer 22 b. The electrolyte layer 23 are, for example, formed bymixing an electrolyte solution, a high molecular weight material, anddimethyl carbonate which is a solvent of this high molecular weightmaterial; applying this mixture on the cathode mixture layer 21 b or theanode mixture layer 22 b; drying it, and volatilizing the solvent.

[0050] After forming the electrolyte layer 23, for example, as shown inFIG. 2, the separator 24, the cathode 21 formed with the electrolytelayer 23, the separator 24, and the anode 22 formed with the electrolytelayer 23 are sequentially layered and wound, and the protective tape 25is, for example, adhered at the outermost circumferential part. In thismanner, the battery element 20 is formed.

[0051] After producing the battery element 20, the exterior members 30 aand 30 b made of e.g. aluminum laminated films are prepared, betweenwhich the battery element 20 is sandwiched. In the reduced-pressureatmosphere, the exterior members 30 a and 30 b are crimped to thebattery element 20, which is enclosed by contacting outer edges of theexterior members 30 a and 30 b by heat anastomoses or the like. Then,for example, the adhesive film 31 is inserted between the cathode leadwire 11 and the anode lead wire 12 and the exterior members 30 a and 30b, and the cathode lead wire 11 and the anode lead wire 12 are projectedfrom the exterior members 30 a and 30 b. In this manner, assembly of thesecondary battery is completed. A shape of the secondary battery is notlimited to the shape shown in FIGS. 1 and 2, and other shapes arecontemplated within the scope of the present invention.

[0052] After assembling the secondary battery, for example, thesecondary battery is heated to higher temperature than normaltemperatures while being uniaxial pressurized. Namely, the batteryelement 20 is heated while being pressurized through the exteriormembers 30 a and 30 b. In this way, the electrolyte solution containedin the electrolyte layer 23 is penetrated in the cathode mixture layer21 b and the anode mixture layer 22 b, so that the adhesion between theelectrolyte layer 23 and the cathode 21 and the anode 22 is raised.Additionally, the adhesion between each electrode active material israised, and the contact resistance of the electrode active material islowered. Through the above processes, the secondary battery iscompleted.

[0053] In the secondary battery, when charge is performed, for example,lithium ions extract from the cathode 21, and are inserted into theanode 22 via the electrolyte layer 23 and the separator 24. Whendischarge is performed, for example, lithium ions extract from the anode22, and are inserted into the cathode 21 via the electrolyte layer 23and the separator 24. In this case, an amount of the binder contained inthe cathode mixture layer 21 b is suppressed to be a small amount, andthe content of the electrode active material is increased. Therefore,internal resistance in the battery is reduced, the charge and dischargecapacity and the charge and discharge cycle life are improved, andparticularly the load characteristics are improved.

[0054] The charge final voltage is about 4.0 V or less in an embodiment.When it exceeds about 4.0 V, the above-mentioned binder contained in thecathode mixture layer 21 b is decomposed, thereby causing lowering ofthe cycle characteristics and load discharge capacity. However, when alithium phosphorous oxide is used as a cathode active material,excellent battery characteristics can be obtained with the charge finalvoltage of about 4.0 V or less.

[0055] As discussed above, in an embodiment, effective amounts of thesynthetic rubber latex adhesive and the thickener are contained in thecathode mixture layer 21 b as a binder. Therefore, the flexible andsmooth cathode 21 can be obtained, and electrode peeling and crackingcan be prevented. Additionally, effective amounts of the syntheticrubber latex adhesive and the thickener, or maleic acid-denaturalizedpolyvinylidene fluoride are contained, so that a content of the binderlowers, the ratio of the cathode active material in the cathode mixturelayer 21 b and the capacity of the cathode 21 can be increased, andelectron transfer in the cathode 21 can be facilitated. Therefore, thecharge and discharge capacity, and the charge and discharge cycle lifecan be improved, and the load characteristics can be improved.

[0056] In particular, in the case where denaturalized amount of maleicacid-denaturalized polyvinylidene fluoride ranges from about 0.1 wt % toabout 0.4 wt %, and part of maleic acid-denaturalized polyvinylidenefluoride is substituted with hexafluoropropylene of about 5 wt % orless, a higher discharge capacity can be obtained, and the cyclecharacteristics and the high load characteristics can be furtherimproved.

[0057] Examples of the present invention, without limitation, will bedescribed below in greater detail with reference to FIGS. 1 and 2.

EXAMPLES 1-1 AND 1-2

[0058] First, as a cathode active material, lithium iron phosphorousoxide (LiFePO₄) as the lithium phosphorous oxide was prepared under thefollowing conditions. Lithium phosphate and phosphorous iron (II) oxideoctahydrate were mixed at an element ratio of lithium:iron=1:1. Ketjenblack powders were added to the mixture, so that its ratio was 10% ofthe whole fired material obtained after firing, thereby obtaining amixed sample. This mixed sample was put in an alumina container, andmilling was made by a planetary ball mill at a weight ratio ofsample:alumina ball that equaled 50%, at a number of revolutions of 250rpm, and for operation time of 10 hours. After that, the mixed samplewas put in a ceramic pot, and was fired in an electric furnace in thenitrogen atmosphere at 600° C. for 5 hours, thereby obtaining a firedmaterial of LiFePO₄ containing a carbon material.

[0059] The cathode mixture was prepared by using the LiFePO₄ as thecathode active material, sufficiently kneading the fired material ofLiFePO₄ containing the carbon material, and ammonium salt polyacrylicacid (PAA) in a planetary mixer, and adding styrene butadiene rubberlatex (SBR) to the mixture. Then, a mass ratio of the fired material ofLiFePO₄ containing the carbon material:ammonium salt polyacrylicacid:styrene butadiene rubber latex was (99−x):1:x. The value of x wasvaried as shown in Examples 1-1 and 1-2 in Table 1. The cathode mixturewas dispersed in N-methyl-2-pyrrolidone, thereby obtaining the cathodemixture slurry. Then, the cathode mixture slurry was uniformly appliedon both sides of the cathode current collector layer 21 a made of astrip-shaped aluminum foil, and dried. After that, compression moldingwas performed by a roller pressing machine, thereby forming thestrip-shaped cathode mixture layer 21 b, which were cut in a given sizeto produce the sheet cathode 21. TABLE 1 Mass ratio of total SBR PAAPVDF amount mass mass mass of Peel strength Binder ratio x ratio ratio ybinder test Example 1-1 SBR + 2 1 — 3 Sufficient PAA Example 1-2 SBR + 41 — 5 Strong PAA Comparative SBR + 1 1 — 2 Insufficient Example 1-1 PAAComparative SBR + 5 1 — 6 Meas- Example 1-2 PAA urement impossibleComparative PVDF — — 3 3 Insufficient Example 1-3 Comparative PVDF — — 66 Sufficient Example 1-4

[0060] Next, mesophase carbon micro beads as the anode active materialand polyvinylidene fluoride as a binder were mixed at a mass ratio of90:10, to thereby prepare an anode mixture. Subsequently,N-methyl-2-pyrrolidone as the solvent was added to this anode mixture,which was then stirred and mixed to obtain an anode mixture slurry.Subsequently, the anode mixture slurry was uniformly applied on bothsides of the anode current collector layer 22 a made of a strip-shapedcopper foil and dried. After that, compression molding was performed bya roller pressing machine to form the strip-shaped anode mixture layer22 b, which were cut in a given size to produce the sheet anode 22.

[0061] Further, LiPF₆ was dissolved in a solvent wherein ethylenecarbonate and propylene carbonate were mixed to produce an electrolytesolution. After that, the electrolyte solution, a high molecular weightmaterial, and dimethyl carbonate as the solvent of this high molecularweight material were mixed and stirred to obtain a gel electrolyte.

[0062] After producing the cathode 21, the anode 22, and theelectrolyte, the cathode lead wire 11 was attached to the cathodecurrent collector layer 21 a, the electrolyte was applied to the cathodemixture layer 21 b, dimethyl carbonate was evaporated, thereby formingthe electrolyte layer 23. Additionally, the anode lead wire 12 wasattached to the anode current collector layer 22 a, the electrolyte wasapplied to the anode mixture layer 22 b, dimethyl carbonate wasevaporated, thereby forming the electrolyte layer 23. Then, a pair ofseparator 24 made of a micro-porous polypropylene film with a thicknessof 9 μm was prepared. The separator 24, the cathode 21, the separator24, and the anode 22 were sequentially layered in this order and wound,and the protective tape 25 was adhered, thereby obtaining the batteryelement 20. The battery elements 20 were produced for Examples 1-1 and1-2 respectively.

[0063] Peel strength tests were conducted for the battery elements 20 ofExamples 1-1 and 1-2 produced as above, based on JIS B 7721. Obtainedresults are shown in Table 1. The peel strength tests in Table 1 wereevaluated as “Strong” when binding force is 7 gf/mm and above;“Sufficient” when binding force is 2 gf/mm and above, and less than 7gf/mm; and “Insufficient” when binding force is less than 2 gf/mm.

[0064] As Comparative Examples 1-1 and 1-2 to be compared with Examples1-1 and 1-2, the battery elements 20 were produced in a manner similarto Examples 1-1 and 1-2, except that the value of x in (99−x):1:x, whichis a mass ratio of the fired material of LiFePO₄ containing a carbonmaterial to ammonium salt polyacrylic acid to styrene butadiene rubberlatex, was varied as shown in Table 1. Further, as Comparative Examples1-3 and 1-4 to be compared with Examples 1-1 and 1-2, the batteryelements 20 were produced in a manner similar to Examples 1-1 and 1-2,except that a binder made of polyvinylidene fluoride (PVDF) was usedinstead of the binder made of ammonium salt polyacrylic acid and styrenebutadiene rubber latex, and the cathode mixture was made at a mass ratioof a fired material of LiFePO₄ containing a carbonmaterial:polyvinylidene fluoride=(100−y):y. The value of y was varied asshown in Comparative Examples 1-3 and 1-4 in Table 1. ComparativeExample 1-4 is a typical example of a binder contained in a conventionalcathode mixture. The peel strength tests were also conducted forComparative Examples 1-1 to 1-4 in a manner similar to Examples 1-1 and1-2. Obtained results are shown in Table 1.

[0065] As evidenced by Table 1, sufficient binding force was obtained inExample 1-1, and strong binding force was obtained in Example 1-2. InComparative Example 1-1, however, sufficient binding force could not beobtained. It is believed, for example, that this result was due to theamount of styrene butadiene rubber latex as the binder that was toosmall. In Comparative Example of binder 1-2, viscosity of the cathodemixture was significantly raised, and application to the cathode currentcollector layer 21 a could not be performed well. It is believed, forexample, that this resulted because the amount of styrene butadienerubber latex as the binder was too great. In Comparative Example 1-3,containing polyvinylidene fluoride of 3 wt %, no binding force wasobserved, and significant peeling was shown at the electrode. InComparative Example 1-4, containing polyvinylidene fluoride of 6 wt %,though sufficient binding force was obtained, the content of the binderwas too great.

[0066] In this regard, it was found that by using ammonium saltpolyacrylic acid and styrene butadiene rubber latex as a binder, andsetting the content of styrene butadiene rubber latex to the range fromabout 2 wt % to about 4 wt %, an amount of binder became smaller, andthe cathode and the secondary battery with sufficient binding force, forexample, high strength could be obtained.

EXAMPLES 2-1 AND 2-2

[0067] The rolled battery elements 20 were produced in a manner similarto Example 1-1, except that the value of z in (98−z):z:2, which is amass ratio of a fired material of LiFePO₄ containing a carbon materialto ammonium salt polyacrylic acid to styrene butadiene rubber latex, wasvaried as shown in Table 2. The peel strength tests were conducted forthe battery elements 20 of Examples 2-1 and 2-2 made as above in similarway to Example 1-1. Obtained results are shown in Table 2. The peelstrength tests in Table 2 were evaluated in a manner similar to Table 1.TABLE 2 Mass ratio SBR PAA of total mass mass amount Peel strengthBinder ratio ratio z of binder test Example 2-1 SBR + PAA 2 0.5 2.5Sufficient Example 2-2 SBR + PAA 2 2.5 4.5 Sufficient Comparative SBR +PAA 2 0.3 2.3 Insufficient Example 2-1 Comparative SBR + PAA 2 3 5Measurement Example 2-2 impossible

[0068] As Comparative Examples 2-1 and 2-2 to be compared with Examples2-1 and 2-2, the battery elements 20 were produced in a manner similarto Examples 2-1 and 2-2, except that the value of z in (98−z):z:2, whichis a mass ratio of the fired material of LiFePO₄ containing a carbonmaterial to ammonium salt polyacrylic acid to styrene butadiene rubberlatex, was varied as shown in Table 2. The peel strength tests were alsoconducted for Comparative Examples 2-1 and 2-2 in a manner similar toExamples 2-1 and 2-2. Obtained results are shown in Table 2.

[0069] As evidenced by Table 2, sufficient binding force was obtained inExamples 2-1 and 2-2. In Comparative Example 2-1, however, the bindingforce was insufficient. It is believed, for example, that this resultedbecause the content of ammonium polyacrylic acid was too small. InComparative Example 2-2, gelation of the cathode mixture wassignificant, so that application to the cathode current collector layer21 a was impossible. It is believed, for example, that this resultedbecause the content of ammonium polyacrylic acid was too much.

[0070] In this regard, it was found that by using ammonium saltpolyacrylic acid and styrene butadiene rubber latex as a binder, andsetting the content of ammonium salt polyacrylic acid to the range fromabout 0.5 wt % to about 2.5 wt %, thickening effect was properly givento the cathode mixture, and sufficient binding force could be obtained.

EXAMPLES 3-1 AND 3-2

[0071] The battery elements 20 were produced in a manner similar toExample 1-1, except that a mass ratio of a fired material of LiFePO₄containing a carbon material to ammonium salt polyacrylic acid tostyrene butadiene rubber latex, was set to 97:1:2. Subsequently, theexterior members 30 a and 30 b made of aluminum laminated films wereprepared, the adhesive film 31 was arranged between the cathode leadwire 11 and the anode lead wire 12 and the exterior members 30 a/30 b,and the battery elements 20 were vacuum packaged. In this manner, thesecondary batteries were assembled.

[0072] Charge and discharge cycle tests and high load discharge testswere conducted for the secondary batteries of Examples 3-1 and 3-2produced as above. Obtained results are shown in Table 3.

[0073] The charge and discharge cycle tests were conducted as follows.First, charging was performed with a constant current of 100 mA untilthe charge final voltage shown in Table 3 was attained. After that,discharging was performed with a constant current of 100 mA until thebattery voltage reached 2.0 V. Charge and discharge were repeated underthe same conditions, and the discharge capacity of the 10th cycle,wherein the charge and discharge capacity settled was measuredrespectively. TABLE 3 Discharge 1000 mA capacity high load CathodeCharge final of the 10^(th) discharge active Binder voltage cyclecapacity material Binder (wt %) (V) (mAh) (mAh) Example 3-1 LiFePO₄SBR + PAA 3 3.6 519 421 Example 3-2 LiFePO₄ SBR + PAA 3 4.0 523 413Comparative LiFePO₄ SBR + PAA 3 4.2 273 138 Example 3-1 ComparativeLiCoO₂ PVDF 6 4.2 485 342 Example 3-2

[0074] High load charge and discharge tests were conducted as follows.First, charging was performed with a constant current of 100 mA untilthe charge final voltage shown in Table 3 was attained. After that,discharging was performed with a constant current of 1000 mA until thebattery voltage reached 2.0 V, and respective discharge capacities weremeasured.

[0075] As to the secondary batteries of Comparative Example 3-1 to becompared with Examples 3-1 and 3-2, made in a manner similar to Example3-1, the charge and discharge cycle tests and the high load charge anddischarge tests were conducted in a manner similar to Examples 3-1 and3-2, except that the charge final voltage was 4.2 V. Obtained resultswere shown in Table 3. As to the secondary batteries of ComparativeExample 3-2 to be compared with Examples 3-1 and 3-2, made in a mannersimilar to Example 3-1, except that LiCoO₂ was used as a cathode activematerial, and polyvinylidene fluoride of 6 wt % was used as a binder ata mass ratio of 94:6, the charge and discharge cycle tests and the highload charge and discharge tests were conducted in a manner similar toExamples 3-1 and 3-2, except that the charge final voltage was 4.2 V.Obtained results are shown in Table 3. The cathode mixture ofComparative Example 3-2 is a conventionally typical example.

[0076] As evidenced by Table 3, according to Examples 3-1 and 3-2, thedischarge capacities of the 10th cycle were high, such as 519 mAh andabove, and the high load discharge capacities of 1000 mA were high, suchas 413 mA and above. Meanwhile, in Comparative Example 3-1, thedischarge capacity of the 10th cycle was 273 mAh and the high loaddischarge capacity was 138 mAh, such as, both capacities were very low.In this regard, it was shown that in Examples 3-1 and 3-2, the cyclecharacteristics and the high load discharge capacity were improved. Itis believed, for example, that when charge OCV (open circuit voltage)has a potential of 4.0 V and under, ammonium salt polyacrylic acid andstyrene butadiene rubber latex were not decomposed, and when itspotential was 4.2 V and above, ammonium salt polyacrylic acid andsynthetic rubber latex were decomposed. In Comparative Example 3-2, thecontent of the binder was twice as large as in Examples of 3-1 and 3-2,and the discharge capacity of the 10th cycle was not low, such as 485mAh, but the high load discharge capacity was low, such as 342 mAh. Inthis regard, it was confirmed that in the case where ammonium saltpolyacrylic acid and styrene butadiene rubber latex were used as abinder, even when the total amount of the binder was decreased,excellent cycle characteristics were obtained, and the high loaddischarge capacity was improved.

[0077] In this regard, it was confirmed that when the charge finalvoltage was not over 4.0 V, ammonium salt polyacrylic acid and styrenebutadiene rubber latex could be used as a binder, so that the content ofthe binder can be lowered, and the cycle characteristics and the highload discharge capacity were improved.

[0078] In Examples 1-1 to 3-2, ammonium salt polyacrylic acid was usedas a thickener. However, the same effect was confirmed when natrium saltpolyacrylic acid, ammonium salt carboxymethyl cellulose, or natrium saltcarboxymethyl cellulose was used.

[0079] In Examples 1-1 to 3-2, LiFePO₄ was used as a cathode activematerial. However, when using other systems whose charge final voltagewas not over 4.0 V, such as LiFe_(0.2)Cu_(0.8)PO₄, LiFe_(0.9)Ti_(0.1).PO₄, LiFe_(0.8)Zn_(0.2) PO₄, LiFe_(0.8)Mg_(0.2)PO₄ and the like as acathode active material, similar effect was confirmed.

[0080] The results of Examples 1-1 to 3-2 demonstrate that by using asynthetic rubber latex adhesive, such as styrene butadiene rubber latex,and a thickener, such as ammonium salt polyacrylic acid, as a binder ofthe cathode 21 in the secondary battery with its charge final voltage ofabout 4.0 V or less, the cycle characteristics and the high loaddischarge capacity can be improved. In this regard, it was found thatthe secondary battery, wherein the charge and discharge capacity, thecapacity maintenance ratio, and the discharge cycle life were improved,particularly the load characteristics was improved, could be obtained.

[0081] In the above examples, descriptions were made on materials forthe synthetic rubber latex adhesive and the thickener with concreteexamples. However, similar results can also be obtained and contemplatedwhen using materials with other suitable structures according to anembodiment of the present invention.

EXAMPLES 4-1 AND 4-2

[0082] The battery elements 20 were produced in a manner similar toExample 1-1, except that a cathode mixture was prepared as follows.First, a cathode mixture was prepared by sufficiently kneading a firedmaterial of LiFePO₄ containing a carbon material, and polyvinylidenefluoride denaturalized by maleic acid, and partly substituted withhexafluoro propylene (HFPr) (hereinafter referred to as HFPr-substitutedmaleic acid-denaturalized PVDF). Here, amount denaturalized by maleicacid in HFPr-substituted maleic acid-denaturalized PVDF was 0.3 wt %,and amount of hexafluoro propylene used for hexafluoro propylenesubstitution (hereinafter referred to as HFPr-substituted amount) was 3wt %. A mass ratio of the fired material of LiFePO₄ containing thecarbon material and the binder comprised of HFPr-substituted maleicacid-denaturalized PVDF was set to (100−p):p. The value of p was changedas shown in Examples 4-1 and 4-2 in Table 4. The peel strength testswere conducted in a manner similar to Example 1-1, for the batteryelements 20 of Examples 4-1 and 4-2 produced as above. Obtained resultsare shown in Table 4. The peel strength tests in Table 4 were evaluatedin a manner similar to Table 1. TABLE 4 Maleic acid- HFPr- Binderdenaturalized substituted mass Peel strength Binder amount (wt %) amount(wt %) ratio p test Example 4-1 HFPr-substituted maleic 0.3 3 0.5Sufficient acid-denaturalized PVDF Example 4-2 HFPr-substituted maleic0.3 3 4 Strong acid-denaturalized PVDF Comparative HFPr-substitutedmaleic 0.3 3 0.3 Insufficient Example 4-1 acid-denaturalized PVDFComparative HFPr-substituted maleic 0.3 3 5 Measurement Example 4-2acid-denaturalized impossible PVDF Comparative PVDF — — 3 InsufficientExample 1-3 Comparative PVDF — — 6 Sufficient Example 1-4

[0083] As Comparative Examples 4-1 and 4-2 to be compared with Examples4-1 and 4-2, the battery elements 20 were produced in a manner similarto Examples 4-1 and 4-2 except that the value of p in (100−p):p, a massratio of a fired material of LiFePO₄ containing a carbon material and abinder made of HFPr-substituted maleic acid-denaturalized PVDF wasvaried as shown in Table 4. The peel strength tests were also conductedfor Comparative Examples 4-1 and 4-2 in a manner similar to Examples 4-1and 4-2. Obtained results are shown in Table 4. As comparative examplesto Examples 4-1 and 4-2, the results of Comparative Examples 1-3 and 1-4are also shown in Table 4. The polyvinylidene fluoride used inComparative Examples 1-3 and 1-4 was neither denaturalized by maleicacid nor substituted with hexafluoro propylene.

[0084] As evidenced by Table 4, sufficient binding force was obtained inExample 4-1, and strong binding force was obtained in Example 4-2. InComparative Example 4-1, however, sufficient binding force could not beobtained. It is believed, for example, this was due because the amountof HFPr-substituted maleic acid-denaturalized PVDF as the binder was toosmall. In Comparative Example 4-2, gelation of the cathode mixture wassignificant, so that application to the cathode current collector layer21 a was not performed well. It is believed, for example, this was duebecause the amount of HFPr-substituted maleic acid-denaturalized PVDF asthe binder was too much. In Comparative Example 1-3 containingpolyvinylidene fluoride of 3 wt %, no binding force was shown, anddrastic peeling appeared at the electrode. In Comparative Example 4-4containing polyvinylidene fluoride of 6 wt %, though sufficient bindingforce was obtained, the content of the binder was too much.

[0085] In this regard, it was found that by using HFPr-substitutedmaleic acid-denaturalized PVDF as a binder and setting a content ofHFPr-substituted maleic acid-denaturalized PVDF ranges from about 0.5 wt% to about 4 wt %, a content of binder became small, and the cathode andthe secondary battery with sufficient binding force, i.e. high strengthwere obtained.

EXAMPLES 5-1 TO 5-2

[0086] The rolled battery elements 20 were produced in a manner similarto Example 4-1, except that an amount denaturalized by maleic acid ofHFPr-substituted maleic acid-denaturalized PVDF was varied as shown inTable 5, and a mass ratio of a fired material of LiFePO₄ containing acarbon material to HFPr-substituted maleic acid-denaturalized PVDF wasset to 98:2. The peel strength tests were conducted for the batteryelements 20 of Examples 5-1 and 5-2 made as above in a manner similar toExample 4-1. Obtained results are shown in Table 5. The peel strengthtests in Table 5 were evaluated in a manner similar to Table 4. TABLE 5Maleic acid- Binder denaturalized HFPr-substituted mass Peel strengthBinder amount (wt %) amount (wt %) ratio p test Example 5-1HFPr-substituted 0.1 3 2 Sufficient maleic acid- denaturalized PVDFExample 5-2 HFPr-substituted 0.4 3 2 Strong maleic acid- denaturalizedPVDF Comparative HFPr-substituted 0.05 3 2 Insufficient Example 5-1maleic acid- denaturalized PVDF Comparative HFPr-substituted 0.5 3 2Measurement Example 5-2 maleic acid- impossible denaturalized PVDF

[0087] As Comparative Examples 5-1 and 5-2 to be compared with Examples5-1 and 5-2, the rolled battery elements 20 were produced in a mannersimilar to Example 4-1, except that an amount denaturalized by maleicacid of HFPr-substituted maleic acid-denaturalized PVDF was varied asshown in Table 5, and a mass ratio of a fired material of LiFePO₄containing a carbon material to HFPr-substituted maleicacid-denaturalized PVDF was set to 98:2. The peel strength tests werealso conducted for Comparative Examples 5-1 and 5-2 in a manner similarto Examples 5-1 and 5-2. Obtained results are shown in Table 5.

[0088] As evidenced by Table 5, sufficient binding force was obtained inExamples 5-1, and strong binding force was obtained in Example 5-2. InComparative Example 5-1, however, the binding force was insufficient andthe cathode 21 with insufficient strength was obtained. It is believed,for example, this resulted because the amount denaturalized by maleicacid was too small. In Comparative Example 5-2, gelation of the cathodemixture was significant, so that application to the cathode currentcollector layer 21 a was impossible. It is believed, for example, thisresulted because the amount denaturalized by maleic acid was too much.

[0089] In this regard, it was found that by using HFPr-substitutedmaleic acid-denaturalized PVDF as a binder and the maleicacid-denaturalized amount of HFPr substituted-maleic acid-denaturalizedPVDF that ranges from about 0.1 wt % to about 0.4 wt %, the cathode withsufficient binding force, such as high strength and the secondarybattery were obtained, and reduction of the amount of binder wassupported.

EXAMPLES 6-1 TO 6-3

[0090] Cathode mixture was prepared by setting a mass ratio of a firedmaterial of LiFePO₄ containing a carbon material, to maleicacid-denaturalized polyvinylidene fluoride (hereinafter referred to asmaleic acid-denaturalized PVDF) to 98:2. The maleic acid-denaturalizedamount of maleic acid-denaturalized PVDF was 0.3 wt %, and amount ofhexafluoro propylene used for hexafluoro propylene substitution(hereinafter referred to as HFPr-substituted amount) was varied as shownin Table 6. Then, the battery elements 20 were formed in a mannersimilar to Example 4-1 except for the above. TABLE 6 1000 mA Dischargehigh HFPr- capacity load substituted Peel of the 10th discharge amountBinder strength cycle capacity (wt %) (wt %) test (mAh) (mAh) Example6-1 5 2 Strong 521 418 Example 6-2 No substitution 2 Strong 509 398Example 6-3 6 2 Strong 295 152

[0091] The peel strength tests were conducted in a manner similar toExample 4-1 for the battery elements 20 of Examples 6-1 to 6-3 made asabove. Obtained results are shown in Table 6. The peel strength tests inTable 6 were evaluated in a manner similar to Table 4.

[0092] Further, the external members 30 a and 30 b made of aluminumlaminated films were prepared, adhesive film 31 was arranged between thecathode lead wire 11 and anode lead wire 12 and the external materials30 a and 30 b, and the battery elements 20 of Examples 6-1 to 6-3produced as above were vacuum packaged. In this way, the secondarybattery was assembled.

[0093] The charge and discharge cycle tests and the high load dischargetests were conducted for the secondary batteries of Examples 6-1 to 6-3made as above, in a manner similar to Examples 3-1 and 3-2. Obtainedresults are shown in Table 6.

[0094] As evidenced by Table 6, strong binding force was obtained in anyof Examples 6-1 to 6-3. In this regard, it was found that by usingmaleic acid-denaturalized PVDF or HFPr-substituted maleicacid-denaturalized PVDF as a binder, a content of the binder becameless, and the cathode and the secondary battery with sufficient bindingforce, such as high strength, were obtained.

[0095] Further, as evidenced by Table 6, according to Examples 6-1 and6-2, the discharge capacities of the 10th cycle were high, such as 509mAh and above, and the high load discharge capacities of 1000 mA werehigh, such as 398 mAh and above. Meanwhile, in Example 6-3, thedischarge capacity of the 10th cycle was 295 mAh and the high loaddischarge capacity was 152 mAh, such as both capacities were low.

[0096] In this regard, it was confirmed that by using HFPr-substitutedmaleic acid-denaturalized PVDF whose HFPr-substituted amount of 5 wt %and under, the cycle characteristics and the high load dischargecapacity were improved.

EXAMPLES 7-1 AND 7-2

[0097] The secondary batteries were produced in a manner similar toExample 6-1, except that HFPr-substituted maleic acid-denaturalizedpolyvinylidene fluoride of 2 wt % whose maleic acid-denaturalized amountwas 0.3 wt % and HFPr-substituted amount was 3 wt %, was used as abinder.

[0098] The charge and discharge cycle tests and the high load dischargetests were conducted for the secondary batteries of Examples 7-1 and 7-2produced as above. Obtained results are shown in Table 7. The charge anddischarge cycle tests and the high load discharge tests were conductedin a manner similar to Examples 6-1 and 6-2, except that the chargefinal voltage was varied as shown in Table 7. TABLE 7 Discharge Chargecapacity of 1000 mA high Cathode final the 10th load discharge activeBinder voltage cycle capacity material Binder (wt %) (V) (mAh) (mAh)Example 7-1 LiFePO₄ HFPr-substituted 2 3.6 522 420 maleic acid-denaturalized PVDF Example 7-2 LiFePO₄ HFPr-substituted 2 4.0 528 417maleic acid- denaturalized PVDF Comparative LiFePO₄ HFPr-substituted 24.2 276 140 Example 7-1 maleic acid- denaturalized PVDF ComparativeLiCoO₂ PVDF 6 4.2 502 342 Example 7-2

[0099] As to the secondary battery of Comparative Example 7-1 to becompared with Examples 7-1 and 7-2, made in a manner similar to Example7-1, the charge and discharge cycle tests and the high load dischargetests were conducted in a manner similar to Examples 7-1 and 7-2, exceptthat the charge final voltage was 4.2 V. Obtained results are shown inTable 7. As Comparative Example 7-2 to be compared with Examples 7-1 and7-2, a secondary battery was produced in a manner similar to ComparativeExample 3-2. Then, the charge and discharge cycle tests and the highload discharge tests were conducted for the secondary battery of thisComparative Example 7-2, in a manner similar to Examples 7-1 and 7-2,except that the charge final voltage was set to 4.2 V Obtained resultsare shown in Table 7.

[0100] As evidenced by Table 7, according to Examples 7-1 and 7-2, thedischarge capacities of the 10th cycle were high, such as 522 mAh andabove, and the high load discharge capacities of 1000 mA were high, suchas 417 mAh and above. Meanwhile, in Comparative Example 7-1, thedischarge capacity of the 10th cycle was 276 mAh and the high loaddischarge capacity was 140 mAh, such as both capacities were very low.In this regard, it was shown that in Examples 7-1 and 7-2, the cyclecharacteristics and the high load discharge capacity were improved. Itis believed, for example, that when charge OCV has a potential of 4.0 Vand under, maleic acid-denaturalized PVDF was not decomposed, and whenits potential was 4.2 V and above, maleic acid-denaturalized PVDF wasdecomposed. In Comparative Example 7-2, the amount of the binder wasthree times as large as in Examples 7-1 and 7-2, and the dischargecapacity of the 10th cycle was not low, such as 502 mAh, but the highload discharge capacity was low, such as 342 mAh. In this regard, it wasconfirmed that in the case where maleic acid-denaturalized PVDF was usedas a binder, even when the total amount of the binder was decreased,excellent cycle characteristics were obtained, and the high loaddischarge capacity was improved.

[0101] Further, it was confirmed that when the charge final voltage wasnot over 4.0 V, maleic acid-denaturalized PVDF could be used as abinder, so that a content of the binder can be lowered, and the cyclecharacteristics and the high load discharge capacity were improved.

[0102] In Examples 4-1 to 7-2, LiFePO₄ was used as a cathode activematerial. The same effect was confirmed when other system with finalcharge voltage of 4.0 V and under, such as LiFe_(0.2)Cu_(0.8)PO₄,LiFe_(0.9)Ti_(0.1)PO₄, LiFe_(0.8)Zn_(0.2)PO₄, LiFe_(0.8)Mg_(0.2)PO₄, andthe like was used as a cathode active material.

[0103] Therefore, it was found that, according to Examples 4-1 to 7-2,by using maleic acid-denaturalized PVDF as a binder of the cathode 21 inthe secondary battery with its charge final voltage of 4.0 V and under,the cathode 21 with high strength could be obtained. Further, it wasfound that the cycle characteristics and the high load dischargecapacity were improved. In particular, it was found that by usingHFPr-substituted maleic acid-denaturalized PVDF as a binder of thecathode 21, the cycle characteristics and the high load dischargecapacity were further improved. In this regard, it was found that thecharge and discharge capacity, the capacity maintenance ratio, and thedischarge cycle life were improved, and in particular, the secondarybattery with improved load characteristics could be obtained.

[0104] As described above, the present invention, in an embodiment,includes lithium iron phosphorus oxide as a cathode active material.However, similar results can be obtained when any suitable cathodeactive material which can realize the secondary battery with its chargefinal voltage of about 4.0 V or less, is used. Further, as describedabove, the present invention, in an embodiment, includes the secondarybatteries using lithium as electrode reactive species. However, thesimilar result can be obtained when any suitable cathode activematerial, which can realize the secondary battery with its charge finalvoltage of 4.0 V and under, is used.

[0105] Though the invention has been described by the embodiment and theexamples, the present invention is not limited to the embodiment andexamples but can be modified in any various and suitable manner. Forexample, the battery element is enclosed inside of the exterior membersmade of aluminum laminated films in the embodiment and examples but, itcan be enclosed inside of the exterior members made of other laminatedfilms in an embodiment.

[0106] Further, the gel electrolyte is used in an embodiment andexamples discussed above, but other electrolytes, for example,electrolyte solution, i.e. the liquid electrolyte, a solid highmolecular weight electrolyte which is obtained by dispersing electrolytesalt in a high molecular weight compound with ion conductivity, a solidinorganic electrolyte or the like can be used in an embodiment.

[0107] For the solid electrolytes as the high molecular weightcompounds, for example, either high molecular weight compounds such aspolyethylene oxide or cross-linked polymer including polyethylene oxide,ester high molecular weight compounds such as polymethacrylate, oracrylate high molecular weight materials or the like can be usedindependently, by mixture, or in copolymerization in molecules in anembodiment. Further, as an inorganic conductor, polycrystal of lithiumnitride, lithium iodide, or lithium hydroxide; a mixture of lithiumiodide and dichromium trioxide, or a mixture of lithium iodide, lithiumsulfide; and diphosphorus subsulfide; and the like in an embodiment canbe used.

[0108] Further, the cathode and the anode are wound in the embodimentand examples described above but, the cathode and the anode can befolded, piled or configured in other suitable manners.

[0109] Further, an example about the secondary battery whose batteryelement with winding structure is enclosed inside the exterior membersis specifically described in an embodiment and examples as discussedabove. However, the invention can be applied to secondary batterieshaving other structures. Additionally, the present invention can besimilarly applied to a cylindrical secondary battery, secondarybatteries having other shapes, such as a coin shape, a button shape, asquare shape or the like.

[0110] As described above, according to the cathode or the batterypursuant to an embodiment of the present invention, effective amounts ofthe synthetic rubber latex adhesive and the thickener are contained inthe cathode mixture layer as a binder, so that flexibility andsmoothness can be improved, and electrode peeling and cracking can beprevented. Further, effective amounts of the synthetic rubber latexadhesive and the thickener, or maleic acid-denaturalized polyvinylidenefluoride is contained, so that a content of the binder is lowered toincrease ratio and capacity of the cathode active material, and electrontransfer in the cathode can be facilitated. Thus, the charge anddischarge capacity and the charge and discharge cycle life can beimproved, and the load characteristics can be improved.

[0111] According to the cathode or the battery pursuant to an embodimentof the present invention, a lithium phosphorous oxide having an olivinestructure is contained as a cathode active material, so that excellentbattery characteristics can be obtained when the charge final voltage is4.0 V and under.

[0112] Further, according to the cathode or the battery pursuant to anembodiment of the present invention, an effective amount of a carbonmaterial is contained in the cathode active material, so that excellentconductivity can be obtained, and the load characteristics and thecharge and discharge capacity can be further improved.

[0113] Furthermore, according to the cathode or the battery pursuant toan embodiment of the present invention, an amount of maleicacid-denaturalized polyvinylidene fluoride ranges from about 0.1 wt % toabout 0.4 wt %, so that electrode peeling can be prevented moreeffectively.

[0114] Additionally, according to the cathode or the battery pursuant toan embodiment of the present invention, a part of maleicacid-denaturalized polyvinylidene fluoride is substituted withhexafluoro propylene and has a substitution ratio of about 5 wt % orless. Therefore, the cycle characteristics and the high load dischargecapacity can be further improved.

[0115] It should be understood that various charges and modifications tothe presently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

The invention is claimed as follows:
 1. A cathode comprising: a cathodemixture layer including a cathode active material and a binder, thebinder including a synthetic rubber latex adhesive and a thickenerwherein the content of the synthetic rubber latex adhesive in thecathode mixture layer ranges from about 2 wt % to about 4 wt %, and thecontent of the thickener in the cathode mixture layer ranges from about0.5 wt % to about 2.5 wt %.
 2. The cathode according to claim 1, whereinthe cathode active material includes a lithium phosphorous oxide thathas an olivine structure.
 3. The cathode according to claim 1, whereinthe cathode mixture layer includes a carbon material as a conductiveagent and wherein the content of the carbon material ranges from about 5wt % to about 12 wt % with respect to the total amount of the cathodeactive material and the carbon material.
 4. A cathode comprising: acathode mixture layer including a cathode active material and a binder,the binder including maleic acid-denaturalized polyvinylidene fluoridewherein the content of the maleic acid-denaturalized polyvinylidenefluoride in the cathode mixture layer ranges from about 0.5 wt % toabout 4 wt %.
 5. The cathode according to claim 4, wherein the amount ofthe maleic acid-denaturalized polyvinylidene fluoride ranges from about0.1 wt % to about 0.4 wt %.
 6. The cathode according to claim 4, whereina part of the maleic acid-denaturalized polyvinylidene fluoride issubstituted with hexafluoro propylene having a substitution ratio thatis about 5 wt % or less.
 7. The cathode according to claim 4, whereinthe cathode active material includes a lithium phosphorous oxide thathas an olivine structure.
 8. The cathode according to claim 4, whereinthe cathode mixture layer contains a conductive agent, and wherein thecontent of the carbon material ranges from about 5 wt % to about 12 wt %with respect to the total amount of the cathode active material and thecarbon material.
 9. A battery comprising: a cathode, the cathodeincluding a cathode mixture layer containing a cathode active material,and a binder including a synthetic rubber latex adhesive and athickener; an anode; and an electrolyte, wherein the content of thesynthetic rubber latex adhesive in the cathode mixture layer ranges fromabout 2 wt % to about 4 wt %, wherein the content of the thickener inthe cathode mixture layer ranges from about 0.5 wt % to about 2.5 wt %,and wherein the battery has a charge final voltage of about 4.0 V orless.
 10. The battery according to claim 9, wherein the cathode activematerial includes lithium phosphorous oxide that has an olivinestructure.
 11. The battery according to claim 9, wherein the cathodemixture layer contains a conductive agent including a carbon material,and wherein the content of the carbon material ranges from about 5 wt %to about 12 wt % with respect to the total amount of the cathode activematerial and the carbon material.
 12. A battery comprising: a cathode,the cathode including a cathode active material and a binder including amaleic acid-denaturalized polyvinylidene fluoride; an anode; and anelectrolyte, wherein the content of the maleic acid-denaturalizedpolyvinylidene fluoride in the cathode mixture layer ranges from about0.5 wt % to about 4 wt %, and wherein the battery has a charge finalvoltage that is about 4.0 V or less.
 13. The battery according to claim12, wherein the amount of the maleic acid-denaturalized polyvinylidenefluoride ranges from about 0.1 wt % to about 0.4 wt %.
 14. The batteryaccording to claim 12, wherein a part of the maleic acid-denaturalizedpolyvinylidene fluoride is substituted with hexafluoro propylene havinga substitution ratio that is about 5 wt % or less.
 15. The batteryaccording to claim 12, wherein the cathode active material includeslithium phosphorous oxide that has an olivine structure.
 16. The batteryaccording to claim 12, wherein the cathode mixture layer contains aconductive agent including a carbon material, and wherein the content ofthe carbon material ranges from about 5 wt % to about 12 wt % withrespect to the total amount of the cathode active material and thecarbon material.